Class and id

Question

what are class and id in css

Answer 1

Goto your CSS course:
CSS Fundamentals
CSS Basics
CSS Rules and Selectors (3/6)
Id and class selector (3rd tab)

study the course contents again:
id and class Selectors

id selectors allow you to style an HTML element that has an id attribute, regardless of their position in the document tree.

Class selectors work in a similar way. The major difference is that IDs can only be applied once per page, while classes can be used as many times on a page as needed.

Answer 2

classes are blocks of 'code' (style) that you can add later to an element by adding the attribute class="myClass"

ids are the same, but, ids are meant to be unique, you can of course have many elements with the sane id and therefore the sane style, but that will interfere with any script on the page

example:
<p id="unique">Yep</p>
<p>nope</p>

#unique { color: red; }

only the #unique p is gonna be red